This invention relates to electrostatographic development apparatus and more specifically to such apparatus wherein substantially uniform contact and substantially uniform nip width is maintained between an imaging member and a developer applicating means.
U.S. Pat. No. 3,084,043 teaches an apparatus and method for the liquid development of electrostatic latent images wherein the liquid developer is presented to a photoreceptor having an electrostatic latent image on its surface. The liquid developer is presented by means of an applicator comprising lands and valleys such that the liquid developer is contained in the valleys out of contact with the photoreceptor, while the surfaces of the lands are in contact with the photoreceptor. In such an arrangement, the liquid developer is attracted from the valleys to the electrostatic latent image in image configuration. A typical example of such an arrangement is an electrostatographic copying apparatus wherein the applicator is a rigid cylindrical member having on its surface a pattern of grooves and ridges which comprise lands and valleys, respectively. A liquid developer is maintained in the valleys below the surface of the lands. The applicator is positioned to come into contact with a photoreceptor bearing on its surface an electrostatic latent image. In a typical electrostatographic copying apparatus the photoreceptor is also a cylindrical member comprising a conductive substrate and a photoconductive coating which supports the electrostatic latent image. The electrostatic latent image is typically produced by first charging the entire surface of the photoreceptor in the dark and then by exposing the charged surface to imagewise radiation.
The portions of the charged photoreceptor surface which are struck by the radiation are discharged, leaving an image pattern of charge on the photoreceptor surface to the non-radiation-struck areas.
The photoreceptor surface bearing the electrostatic latent image and the applicator are brought into moving contact during which the liquid developer is drawn to the photoreceptor from the valleys of the applicator roller by the charges which form the electrostatic latent image. Typically the image is then transferred to an image receiving member such as paper by pressure contact between the photoreceptor and a roller.
Although both of the surfaces may be flat, it is more common for at least one of the surfaces to be arcuate to facilitate the moving of the applicator past sequential points on the photoreceptor while the two are in contact. In compact electrostatographic copying devices the surfaces are typically small diameter cylinders to facilitate the cooperative movement of the surfaces in a confined space. Such movement typically occurs at speeds of about four inches per second, although moving contact resulting in the transfer of liquid developer from the applicator to the photoreceptor occurs at speeds ranging generally from about two to about 70 inches per second.
Although satisfactory visible and recognizable images can be produced by such an apparatus and method, they have frequently been found to lack uniform density. Typically areas of the image which have the same shade of color or density in the original have areas of varying density in the developed image and final copy. Such typical characteristics of the developed image are generally considered unsatisfactory, not pleasing to the eye, and are indices of unacceptable copy quality.
The use of a deformable surface, as either the applicator surface or the photoreceptor surface in such an electrostatographic development apparatus or method when at least one of such surfaces is arcuate provides substantially uniform contact and a substantially uniform nip width between the surfaces.
It has been found that substantially uniform contact between the surfaces is achieved whenever the gap distance between adjacent portions of the surfaces while they are maintained in contact is less than about 0.0005 inch along the line of tangency between the surfaces. In one embodiment a rigid applicator surface has an overall variation along its line of tangency with the photoreceptor of not more than about 0.002 inch and a variation from land to land of not more than about 0.0005 inch. A deformable photoreceptor having a hardness of about 30.degree. as measured on a Shore A Durometer contacts the land surfaces. The gap between the deformable photoreceptor surface and the lands of the rigid applicator surface in such an arrangement is maintained at about 0.0005 inch or less to provide substantial contact between the surfaces.
The nip width in that exemplary embodiment is the zone of substantial contact between the two surfaces. Substantially uniform nip width, is achieved whenever the zone of substantial contact between the surfaces varies not more than about ten fold. A preferred nip width variation is about .+-. 50%. In the embodiment described just above, the photoreceptor is the deformable member. However, it is to be understood that the applicator may be the deformable member. The deformable member may have a hardness of up to about 90.degree. (as measured on a Shore A durometer). For producing copies of consistent sharpness and clarity a preferred hardness is from about 40.degree. to about 70.degree., and optimum print quality is achieved from about 50.degree. to about 60.degree.. An important aspect of the arrangement described above is the ability of the deformable surface to maintain its functional integrity during deformation. That is, the deformable member, whether the applicator or the photoreceptor continues to provide its intended function during deformation.
The establishing of a substantially uniform nip width and of substantial contact as the surfaces move in operative contact provides substantially uniform periods of time during which the liquid developer is able to move from the applicator valleys to the photoreceptor surface across a substantially uniform gap of less than 0.0005 inch. Thus, substantially uniform amounts of liquid developer are transferred to the photoreceptor in response to substantially equally charged portions of the image.
Although it is desired to maintain substantially uniform contact and substantially nip width between an imaging member and a developer applicator means adequate an expedient method to provide same have not as yet been successfully devised.
It is therefore an object of this invention to provide an electrostatographic development apparatus devoid of the above-noted deficiencies.
Another object of this invention is to provide a novel electrostatographic development apparatus wherein substantially uniform contact and substantially nip width is maintained between an imaging member and the developer applicator means to provide satisfactory development.
These and other objects are obtained in accordance with the apparatus of the instant invention generally speaking by providing a roller which is sufficiently resilient across its functional surface to maintain substantial contact and a substantially uniform nip width along its line of axial tangency with a cooperating surface, said roller comprising a rigid central member, a flexible sleeve, and an elastic substance positioned between the flexible sleeve and the rigid member, in which the elastic substance is axially compressed.
Photoreceptor members and applicators in the form of resilient rollers formed in accordance with the present invention provide for the development of electrostatic latent images of similar or equal potential by the application or deposition of substantially equal amounts of developer per respective image potentials. Preferably the nip width variation in cooperating use is not more than about ten fold, the resilient roller has a linear variation along its line of tangency of not more than about 0.002 inches and a variation from land to land of not more than about 0.0005 inches, and its surface hardness is in the range 40.degree. to 70.degree. (as measured on a Shore A durometer), and optimumly of from about 50.degree. to about 60.degree.. Such preferred features provide developed images having densities corresponding to those of the original image.